Echinoderms (Phylum Echinodermata) are a phylum of marine animals (including sea stars). Echinoderms are found at every ocean depth, and includes about 7,000 living species.
The Echinoderms are important both biologically and geologically: biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.
Two main subdivisions of Echinoderms are traditionally recognised: the more familiar, motile Eleutherozoa, which encompasses the Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars) and Holothuroidea (sea cucumbers); and the sessile Pelmatazoa, which consists of the crinoids. Some crinoids, the feather stars, have secondarily re-evolved a free-living lifestyle.
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